Incorrect and variable IP packet checksum value

Question

GOAL: crafting a packet from zero and send it.

PROBLEM: after I enabled "validate checksum" on wireshark I noticed that the expected checksum value should be 0xa85d but the checksum function I'm using computes a different value each time. Why the checksum is changing if the packet is always the same? What is the cause of the checksum mismatch between the one I compute and the one expected by wireshark? Thanks

#include <sys/socket.h>
#include <linux/if_packet.h>
#include <net/ethernet.h> /* the L2 protocols */
#include <netinet/ip.h> 
#include <netinet/udp.h> 
#include <stdio.h>
#include <arpa/inet.h> /* htons */
#include <sys/ioctl.h> 
#include <net/if.h> /* ifreq */
#include <string.h> 
#include <stdlib.h>

#include <linux/seccomp.h> /* seccomp_data */

#define BUF_SIZE 1024 



// https://gist.github.com/leonid-ed/909a883c114eb58ed49f
unsigned short csum(unsigned short *buf, int nwords)
{
  unsigned long sum;
  for(sum=0; nwords>0; nwords--)
    sum += *buf++;
  sum = (sum >> 16) + (sum &0xffff);
  sum += (sum >> 16);
  return (unsigned short)(~sum);
}

int main(){

    const char IF[] = "lo"; // modify to change interface
    int sockfd, ifindex, tx_len=sizeof(struct ether_header)+sizeof(struct iphdr)+sizeof(struct udphdr);
    struct ifreq ifr;
    size_t if_name_len;
    char buf[BUF_SIZE];
    struct ether_header *eh = (struct ether_header *) buf;
    struct iphdr *iph = (struct iphdr *) (buf + sizeof(struct ether_header));
    struct udphdr *udph = (struct udphdr *) (buf + sizeof(struct ether_header) + sizeof(struct iphdr));
    unsigned char *data = buf + sizeof(struct ether_header) + sizeof(struct iphdr) + sizeof(struct udphdr);
    u_int16_t src_port, dst_port;
    struct sockaddr_ll dst_addr;
    struct seccomp_data sec_payload;
    const char dmac[] = {0xaa, 0xbb, 0xcc, 0xdd, 0xee, 0xff};
    const char smac[] = {0x00, 0xd0, 0x56, 0xf2, 0xb5, 0x12};

    // create raw socket to send/receive ethernet frames that can transport all protocols
    if ((sockfd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))) == -1) {
        perror("socket");
    }

    // get interface name length
    if_name_len = strlen(IF);
    if(if_name_len < IF_NAMESIZE) {
        strncpy(ifr.ifr_name, IF, strlen(IF));
        ifr.ifr_name[if_name_len]=0;
    }

    // get the interface index number
    if(ioctl(sockfd, SIOCGIFINDEX, &ifr) == -1){
        perror("ioctl");
    }
        ifindex = ifr.ifr_ifindex;

    // build ethernet header    
        memcpy(eh->ether_dhost, dmac, ETHER_ADDR_LEN);  
        memcpy(eh->ether_shost, smac, ETHER_ADDR_LEN);  
    eh->ether_type = htons(ETH_P_IP);

    // add a struct seccomp_data as data 
        memset(&sec_payload,5,sizeof(struct seccomp_data)); 
    data = (char*)malloc(sizeof(struct seccomp_data));
    memcpy(data, (const unsigned char *)&sec_payload, sizeof(struct seccomp_data));

    int i;
    for(i=0;i<sizeof(sec_payload);i++){
        buf[tx_len++] = data[i];
        printf("%02X ",data[i]);
    }

    // build ip header
    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = htons(sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(sec_payload));
    iph->id = htons(54321);
    iph->frag_off = 0;
    iph->ttl = 64;
    iph->protocol = IPPROTO_UDP;
    iph->check = 0;
    iph->saddr = inet_addr("127.0.0.1");
    iph->daddr = inet_addr("127.0.0.1");

    // build udp header 
    udph->source = htons(8090);
    udph->dest = htons(8091);
    udph->len = htons(sizeof(struct udphdr) + sizeof(sec_payload));

    iph->check = csum((unsigned short *)buf,  sizeof(struct iphdr) + sizeof(struct udphdr) + sizeof(sec_payload));

    memset(&dst_addr,0,sizeof(struct sockaddr_ll)); 
    dst_addr.sll_ifindex = ifr.ifr_ifindex; 
    dst_addr.sll_halen = ETH_ALEN;
    memcpy(dst_addr.sll_addr, dmac, ETH_ALEN);

        if (sendto(sockfd, buf, tx_len, 0, (struct sockaddr*)&dst_addr, sizeof(struct sockaddr_ll)) < 0)
        printf("Send failed\n");
    return 0;
}